This invention relates to a method of manufacturing thin sheets of semiconductor material having an oriented crystal structure. Semiconductor material of high purity and crystalline perfection is desirable for a plurality of purposes. In many cases, a monocrystalline sheet of material is particularly desirable. This invention is directed to the manufacture of material useful in producing high quality photovoltaic cells at low cost.
Much of the power used by spacecraft and deep space probes is provided by photovoltaic solar energy cells made from thin silicon single crystal slices. In order to provide high purity single crystals of silicon for photovoltaic cells, this silicon is usually grown in single crystal ingots which are drawn slowly out of a hot melt of pure silicon. Wafers of silicon are then sliced from this single crystal ingot, for example with a diamond saw. These somewhat thin wafers are then further processed by conventional technology for use in photovoltaic cells. However, the diamond saw cut widths represent losses of valuable material. Also, the polishing step involves further costs, time and losses. Finally, the resultant polished oriented silicon wafer is much thicker than optically desirable.
Another method of forming layers of silicon materials is to deposit a layer from the vapor phase upon a support. The crystal structure of the layer may then be improved by subsequent melting of the layer. It has also been proposed to improve the structure of a layer of semiconductor material on a support by zone melting in which a molten zone is formed and moved through the layer of material.
Yet another approach to the problem is to pull out a thin capillary film of silicon from a melt, this either by the making of a wide ribbon or by the production of an oval or circular section. These capillary film growth methods are difficult because of crystal pulling problems and/or the subsequent handling problems with such thin, solid silicon films.
All of the foregoing methods are subject to limitations, particularly as to the expense involved in forming a layer of semiconductor material and also the limited dimensions and geometry of material which can be formed. A method for making large, thin sheets of semiconductor material having a highly oriented or monocrystalline structure is much to be desired. The reason for this thin films choice is that the cell resistance must be minimized, and thick sections are not optically necessary.
One approach to the manufacture of large sheets of semiconductor material is described in U.S. Pat. No. 3,382,114. This patent describes a method for manufacturing thin semiconductor plates using a zone melting treatment in which the semiconductor material is provided as a powder layer on a support. The upper surface of the powder layer is zone melted while separated from the support by part of the powder layer to reduce contamination. The material is then polished to provide a thin layer of crystalline semiconductor material.
While the above methods have been successful to varying degrees, prior to this invention there was no suitable method available for production of large, thin, oriented sheets of silicon or other semiconductor materials.
Accordingly, it is an object of this invention to provide a melting-orientation process for forming a large sheet of silicon suitable for use in a photovoltaic solar cell by the use of a raster scanning electron beam.
It is a further object to provide a method of making large sheets of oriented crystal semiconductor material of particular geometric designs, both from silicon and from other semiconducting metals and alloys.